Functional reconstitution of an ABC transporter in nanodiscs for use in electron paramagnetic resonance spectroscopy

How to investigate interactions between membrane proteins and ligands by solid-state NMR

Solid-state NMR is an established method for biophysical studies of membrane proteins within the lipid bilayers and an emerging technique for structural biology in general. In particular magic angle sample spinning has been found to be very useful for the investigation of large membrane proteins and their interaction with small molecules within the lipid bilayer. Using a number of examples, we illustrate and discuss in this chapter, which information can be gained and which experimental parameters need to be considered when planning such experiments. We focus especially on the interaction of diffusive ligands with membrane proteins.

Nanodiscs in the studies of membrane-bound cytochrome P450 enzymes

Cytochromes P450 from eukaryotes and their native redox partners cytochrome P450 reductases both belong to the class of monotopic membrane proteins containing one transmembrane anchor. Incorporation into the lipid bilayer significantly affects their equilibrium and kinetic properties and plays an important role in their interactions. We describe here the detailed protocols developed in our group for the functional self-assembly of mammalian cytochromes P450 and cytochrome P450 reductases into Nanodiscs with controlled lipid composition. The resulting preparations are fully functional, homogeneous in size, composition and oligomerization state of the heme enzyme, and show an improved stability with respect to P420 formation. We provide a brief overview of applications of Nanodisc technology to the biophysical and biochemical mechanistic studies of cytochromes P450 involved in steroidogenesis, and of the most abundant xenobiotic-metabolizing human cytochrome P450 CYP3A4.

Nanodiscs as a new tool to examine lipid-protein interactions

Nanodiscs are self-assembled discoidal fragments of lipid bilayers 8-16 nm in diameter, stabilized in solution by two amphipathic helical scaffold proteins. As stable and highly soluble membrane mimetics with controlled lipid composition and ability to add affinity tags to the scaffold protein, nanodiscs represent an attractive model system for solubilization, isolation, purification, and biophysical and biochemical studies of membrane proteins. In this chapter we overview various approaches to structural and functional studies of different classes of integral membrane proteins such as ion channels, transporters, GPCR and other receptors, membrane enzymes, and blood coagulation cascade proteins which have been incorporated into nanodiscs. We outline the advantages provided by homogeneity, ability to control oligomerization state of the target protein and lipid composition of the bilayer. Special attention is paid to the opportunities afforded by nanodisc system for the detailed studies of the role of different lipid properties and protein-lipid interactions in the functional behavior of membrane proteins.

Cell-free protein synthesis of membrane (1,3)-β-d-glucan (curdlan) synthase: co-translational insertion in liposomes and reconstitution in nanodiscs

A membrane-embedded curdlan synthase (CrdS) from Agrobacterium is believed to catalyse a repetitive addition of glucosyl residues from UDP-glucose to produce the (1,3)-β-d-glucan (curdlan) polymer. We report wheat germ cell-free protein synthesis (WG-CFPS) of full-length CrdS containing a 6xHis affinity tag and either Factor Xa or Tobacco Etch Virus proteolytic sites, using a variety of hydrophobic membrane-mimicking environments. Full-length CrdS was synthesised with no variations in primary structure, following analysis of tryptic fragments by MALDI-TOF/TOF Mass Spectrometry. Preparative scale WG-CFPS in dialysis mode with Brij-58 yielded CrdS in mg/ml quantities. Analysis of structural and functional properties of CrdS during protein synthesis showed that CrdS was co-translationally inserted in DMPC liposomes during WG-CFPS, and these liposomes could be purified in a single step by density gradient floatation. Incorporated CrdS exhibited a random orientation topology. Following affinity purification of CrdS, the protein was reconstituted in nanodiscs with Escherichia coli lipids or POPC and a membrane scaffold protein MSP1E3D1. CrdS nanodiscs were characterised by small-angle X-ray scattering using synchrotron radiation and the data obtained were consistent with insertion of CrdS into bilayers. We found CrdS synthesised in the presence of the Ac-AAAAAAD surfactant peptide or co-translationally inserted in liposomes made from E. coli lipids to be catalytically competent. Conversely, CrdS synthesised with only Brij-58 was inactive. Our findings pave the way for future structural studies of this industrially important catalytic membrane protein.

A step-by-step method for the reconstitution of an ABC transporter into nanodisc lipid particles

The nanodisc is a discoidal particle (~ 10-12 nm large) that trap membrane proteins into a small patch of phospholipid bilayer. The nanodisc is a particularly attractive option for studying membrane proteins, especially in the context of ligand-receptor interactions. The method pioneered by Sligar and colleagues is based on the amphipathic properties of an engineered highly a-helical scaffold protein derived from the apolipoprotein A1. The hydrophobic faces of the scaffold protein interact with the fatty acyl side-chains of the lipid bilayer whereas the polar regions face the aqueous environment. Analyses of membrane proteins in nanodiscs have significant advantages over liposome because the particles are small, homogeneous and water-soluble. In addition, biochemical and biophysical methods normally reserved to soluble proteins can be applied, and from either side of the membrane. In this visual protocol, we present a step-by-step reconstitution of a well characterized bacterial ABC transporter, the MalE-MalFGK2 complex. The formation of the disc is a self-assembly process that depends on hydrophobic interactions taking place during the progressive removal of the detergent. We describe the essential steps and we highlight the importance of choosing a correct protein-to-lipid ratio in order to limit the formation of aggregates and larger polydisperse liposome-like particles. Simple quality controls such as gel filtration chromatography, native gel electrophoresis and dynamic light scattering spectroscopy ensure that the discs have been properly reconstituted.

ABC transporters, mechanisms and biology: an overview

This chapter concentrates mainly on structural and mechanistic aspects of ABC (ATP-binding cassette) transporters and, as an example of the physiological significance of these proteins, on lipid transport, vitally important for human health. The chapter considers those aspects of ABC transporter function that appear reasonably well established, those that remain controversial and what appear to be emerging themes. Although we have seen dramatic progress in ABC protein studies in the last 20 years, we are still far from a detailed molecular understanding of function. Nevertheless two critical steps - capture and release of allocrites (transport substrates) involving a binding cavity in the membrane domain, and hydrolysis of ATP by the NBD (nucleotide-binding domain) dimer - are now described by persuasive and testable models: alternating access, and sequential firing of catalysis sites respectively. However, these need to be tested rigorously by more structural and biochemical studies. Other aspects considered include the level at which ATP binding and dimer activation are controlled, the nature of the power stroke delivering mechanical energy for transport, and some unexpected and intriguing differences between importers and exporters. The chapter also emphasizes that some ABC transporters, although important for elimination of toxic compounds (xenobiotics), are also increasingly seen to play crucial roles in homoeostatic regulation of membrane biogenesis and function through translocation of endogenous allocrites such as cholesterol. Another emerging theme is the identification of accessory domains and partners for ABC proteins, resulting in a corresponding widening of the range of activities. Finally, what are the prospects for translational research and ABC transporters?

DEER distance measurements on proteins

Distance distributions between paramagnetic centers in the range of 1.8 to 6 nm in membrane proteins and up to 10 nm in deuterated soluble proteins can be measured by the DEER technique. The number of paramagnetic centers and their relative orientation can be characterized. DEER does not require crystallization and is not limited with respect to the size of the protein or protein complex. Diamagnetic proteins are accessible by site-directed spin labeling. To characterize structure or structural changes, experimental protocols were optimized and techniques for artifact suppression were introduced. Data analysis programs were developed, and it was realized that interpretation of the distance distributions must take into account the conformational distribution of spin labels. First methods have appeared for deriving structural models from a small number of distance constraints. The present scope and limitations of the technique are illustrated.

Reconstitution of respiratory oxidases in membrane nanodiscs for investigation of proton-coupled electron transfer

The function of membrane-bound transporters is commonly affected by the milieu of the hydrophobic, membrane-spanning part of the transmembrane protein. Consequently, functional studies of these proteins often involve incorporation into a native-like bilayer where the lipid components of the membrane can be controlled. The classical approach is to reconstitute the purified protein into liposomes. Even though the use of such liposomes is essential for studies of transmembrane transport processes in general, functional studies of the transporters themselves in liposomes suffer from several disadvantages. For example, transmembrane proteins can adopt two different orientations when reconstituted into liposomes, and one of these populations may be inaccessible to ligands, to changes in pH or ion concentration in the external solution. Furthermore, optical studies of proteins reconstituted in liposomes suffer from significant light scattering, which diminishes the signal-to-noise value of the measurements. One attractive approach to circumvent these problems is to use nanodiscs, which are phospholipid bilayers encircled by a stabilizing amphipathic helical membrane scaffold protein. These membrane nanodiscs are stable, soluble in aqueous solution without detergent and do not scatter light significantly. In the present study, we have developed a protocol for reconstitution of the aa(3)- and ba(3)-type cytochrome c oxidases into nanodiscs. Furthermore, we studied proton-coupled electron-transfer reactions in these enzymes with microsecond time resolution. The data show that the nanodisc membrane environment accelerates proton uptake in both oxidases.

Conformational transitions in the membrane scaffold protein of phospholipid bilayer nanodiscs

Phospholipid bilayer nanodiscs are model membrane systems that provide an environment where membrane proteins are highly stable and monodisperse without the use of detergents or liposomes. Nanodiscs consist of a discoidal phospholipid bilayer encircled by two copies of an amphipathic alpha helical membrane scaffold protein, which is modeled from apolipoprotein A-1. Hydrogen exchange mass spectrometry was used to probe the structure and dynamics of the scaffold protein in the presence and absence of lipid. On nanodisc self-assembly, the entire scaffold protein gained significant protection from exchange, consistent with a large, protein-wide, structural rearrangement. This protection was short-lived and the scaffold protein was highly deuterated within 2 h. Several regions of the scaffold protein, in both the lipid-free and lipid-associated states, displayed EX1 unfolding kinetics. The rapid deuteration of the scaffold protein and the presence of correlated unfolding events both indicate that nanodiscs are dynamic rather than rigid bodies in solution. This work provides a catalog of the expected scaffold protein peptic peptides in a nanodisc-hydrogen exchange mass spectrometry experiment and their deuterium uptake signatures, data that can be used as a benchmark to verify correct assembly and nanodisc structure. Such reference data will be useful control data for all hydrogen exchange mass spectrometry experiments involving nanodiscs in which transmembrane or lipid-associated proteins are the primary molecule(s) of interest.

Receptor-transporter interactions of canonical ATP-binding cassette import systems in prokaryotes

ATP-binding cassette (ABC) transport systems mediate the translocation of solutes across biological membranes at the expense of ATP. They share a common modular architecture comprising two pore-forming transmembrane domains and two nucleotide binding domains. In prokaryotes, ABC transporters are involved in the uptake of a large variety of chemicals, including nutrients, osmoprotectants and signal molecules. In pathogenic bacteria, some ABC importers are virulence factors. Canonical ABC import systems require an additional component, a substrate-specific receptor or binding protein for function. Interaction of the liganded receptor with extracytoplasmic loop regions of the transmembrane domains initiate the transport cycle. In this review we summarize the current knowledge on receptor-transporter interplay provided by crystal structures as well as by biochemical and biophysical means. In particular, we focus on the maltose/maltodextrin transporter of enterobacteria and the transporters for positively charged amino acids from the thermophile Geobacillus stearothermophilus and Salmonella enterica serovar Typhimurium.

Dynamics of alpha-helical subdomain rotation in the intact maltose ATP-binding cassette transporter

ATP-binding cassette (ABC) transporters are powered by a nucleotide-binding domain dimer that opens and closes during cycles of ATP hydrolysis. These domains consist of a RecA-like subdomain and an α-helical subdomain that is specific to the family. Many studies on isolated domains suggest that the helical subdomain rotates toward the RecA-like subdomain in response to ATP binding, moving the family signature motif into a favorable position to interact with the nucleotide across the dimer interface. Moreover, the transmembrane domains are docked into a cleft at the interface between these subdomains, suggesting a putative role of the rotation in interdomain communication. Electron paramagnetic resonance spectroscopy was used to study the dynamics of this rotation in the intact Escherichia coli maltose transporter MalFGK(2). This importer requires a periplasmic maltose-binding protein (MBP) that activates ATP hydrolysis by promoting the closure of the cassette dimer (MalK(2)). Whereas this rotation occurred during the transport cycle, it required not only trinucleotide, but also MBP, suggesting it is part of a global conformational change in the transporter. Interaction of AMP-PNP-Mg(2+) and a MBP that is locked in a closed conformation induced a transition from open MalK(2) to semiopen MalK(2) without significant subdomain rotation. Inward rotation of the helical subdomain and complete closure of MalK(2) therefore appear to be coupled to the reorientation of transmembrane helices and the opening of MBP, events that promote transfer of maltose into the transporter. After ATP hydrolysis, the helical subdomain rotates out as MalK(2) opens, resetting the transporter in an inward-facing conformation.